Eocene Paleoseismic Record of the Green River Formation, Fossil Basin, Wyoming, U.S.A.: Implications of Synsedimentary Deformation Structures In Lacustrine Carbonate Mudstones

Abstract

Laterally extensive intervals containing sedimentary deformation features are identified in outcrops of lacustrine deposits of the Eocene (, 52-51 Ma) Green River Formation in Fossil Basin, southwestern Wyoming. Fossil Basin is the smallest of the paleo-lakes in the Green River system and is located in the Sevier Fold and Thrust Belt, which was tectonically active during deposition. Deformation structures include convolute lamination, load structures, hybrid brittle-ductile features, sedimentary dikes, microfaults, fluid-escape features such as cusps and pipes, oil-shale breccias, and mass-transport deposits. In most cases these structures are hosted by finely laminated, variably organic-rich carbonate mudstones that accumulated in a low-energy profundal lacustrine environment. Deformed intervals range from a few centimeters to 5 m in thickness and are bound above and below by undeformed strata of the same facies, which demonstrates the early post-depositional timing of deformation. The distribution of these features indicates a primary control exerted by sediment rheology, resulting from variation in grain size, clay and organic content, and degree of lithification, which in turn were governed by the depositional environment and early diagenesis.Based on their morphological attributes, lateral extent, and recurrence at different stratigraphic levels, along with their sedimentary environment and the tectonic setting of the basin, the deformation structures are interpreted to be ''seismites,'' i.e., the result of stresses induced by syndepositional earthquakes, and other potential trigger agents are rejected. Moreover, stratigraphic relationships indicate that these small-scale deformation features as well as the mass-transport deposits, formed on a virtually flat lake floor, ruling out instability caused by the gravitational regime. Syndepositional deformation in Fossil Basin points to tectonic activity along the nearby Absaroka and/or Hogsback thrusts and related fault systems. Seismites point to possible tectonic forcing as a mechanism for changes in bathymetry and sedimentation in the Green River Formation and, consequently, demonstrate their potential for elucidating the tectonic history of other lacustrine and marine basins.

Sedimentation, earthquakes, and tsunamis in a shallow, muddy epeiric sea: Grinnell Formation (Belt Supergroup, ca. 1.45 Ga), western North America

“…Instead, deformation was clearly intrastratal, having formed under very shallow burial conditions (Pratt, 1998a). Other examples of crack arrays filled with upward-injected material, as well as small sandstone dikes exhibiting either or both upward and downward injection into enclosing muds, have also been interpreted as seismites, caused by earthquake shaking (e.g., Pratt, 1998a;Törő and Pratt, 2015a, 2015bLeitner et al, 2017). There is no other obvious mechanism to simultaneously dewater muddy sediment and liquefy and mobilize silt and sand, as well as, in many cases, cause folding of the crack fills and brecciation of the matrix during the same or a subsequent event.…”

Sedimentary record of seismic events in the Eocene Green River Formation and its implications for regional tectonics on lake evolution (Bridger Basin, Wyoming)

“…The lack of truncation by erosive lake floor processes and the common gradual transition into undeformed beds both upwards and downwards point to intrastratal deformation. Soft-sediment deformed oil shales in the Green River Formation have been attributed to mass wasting processes, such as slumps (e.g., Tänavsuu-Milkeviciene and Sarg, 2012), and in-situ processes (Törő and Pratt, 2015a, b). Due to the limited observations, both scenarios might be plausible, but the absence of evidence for a single transport direction and the inferred intrastratal origin favours the latter.…”

“…Deformed organic-poor silty mudstone and tuff interbeds, hosted by undeformed laminated oil shale, indicate their higher liquefaction potential. Similarly, muddy interbeds in the Wilkins Peak Member generally show chaotic deformation and disruption pointing to lower resistance to liquefaction (e.g., Berra and Felletti, 2011) or thixotropic behaviour (Picard and High, 1972;Chen and Lee, 2013;Törő and Pratt, 2015a). Coarser grained, silt-and sand-rich intervals underlying plastically deformed beds, in turn, typically show the predominance of brittle structures, suggesting a more consolidated or confined state.…”

“…Lake deposits typically consist of rheologically susceptible, heterolithic sediments deposited in an overall low-energy setting (Renaut and Gierlowski-Kordesch, 2010), which increases the potential for preservation of deformation features and eliminates aseismic trigger mechanisms specific to other depositional environments (e.g., Sims, 1975;Moretti and Sabato, 2007). The sedimentary record of lacustrine basins has therefore been used successfully in paleoseismological studies of Quaternary deposits (e.g., Doig, 1986Doig, , 1990Marco et al, 1996;Chapron et al, 1999;Rodríguez-Pascua et al, 2010;Beck, 2011;Strasser et al, 2013), and to identify tectonically active phases in older successions (e.g., Moretti and Sabato, 2007;Berra and Felletti, 2011;Törő and Pratt, 2015a, b). Embracing the stratigraphic record of sedimentary deformation as a valid tectonic archive in low-energy lacustrine and marine strata opens the door to new avenues of appreciation of such basins and the rheological attributes of their sediments (e.g., Pratt, 1998bPratt, , 2001Martín-Chivelet et al, 2011;El Taki and Pratt, 2012;Törő and Pratt, 2015a).…”

“…Although the sedimentology of the Green River Formation in the Greater Green River Basin or in adjacent Colorado and Utah has been studied extensively, only a few have recorded the presence of sedimentary deformation features and these have done so only in passing (e.g., Bradley, 1930;Picard, 1966;Dyni and Hawkins, 1981;Smoot, 1983). Sedimentary deformation features therefore comprise an essentially unexplored aspect of the geology of the Green River Formation (Törő et al, 2015;Törő and Pratt, 2015a, b). Törő et al (2015)…”